Polycrystalline vanadium oxide nanosheets

ABSTRACT

Improved V 2 O 5  materials are disclosed herein in the form of 2D leaf-like nanosheets. Methods of forming the V 2 O 5  nanosheets and batteries (e.g., lithium-ion) incorporating the V 2 O 5  nanosheets are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 61/722,709, filed Nov. 5, 2012, which is expressly incorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with U.S. government support under CMMI-10300048, awarded by the National Science Foundation. The U.S. Government has certain rights in the invention.

BACKGROUND

Energy storage technology is incontrovertibly one of the great challenges in the modern society facing environmental and ecological concerns, and the lithium ion battery is regarded as one of the most important energy storage devices due to its extensive applications in many areas including portable electronic devices, electric vehicles and implantable medical devices. As the heart of clean energy devices, the development of energy storage materials holds the key to the new generation of energy storage devices in the 21^(st) century. Nanostructured materials have attracted increasing interests in the field of energy materials due to superior electrochemical properties benefited from the unique nanostructure, such as nanoscale dimension, high surface area and large structural freedom which could provide high energy and power density while holding the mechanical integrity and chemical stability after many intercalation/deintercalation cycles.

Vanadium oxide is a multi-functional material which has extensive applications in various fields. Since its first investigation as a battery material for lithium ion batteries over 40 years ago, it has been discovered that during Li⁺ ions intercalation vanadium pentoxide (V₂O₅) possesses high specific electrochemical capacity (theoretical capacity 450 mA h g⁻¹) with four phase transitions which involves five successive phases of Li_(x)V₂O₅ (0<x<3): α (x<0.01), ε (0.35<x<0.7), δ (0.9<x≤1), γ (0<x≤2) and the irreversible ω (x>2). Although the Li-ion intercalation voltage is lower than LiCoO₂ or LiMn₂O₄, V₂O₅ has still been regarded as one of the most popular cathode candidates for Li ion batteries due to these advantages: V₂O₅ provides higher energy and power density than LiCoO₂ and LiFePO₄, is easier and more controllable fabrication method than LiMO₂ (M=Ni, Mn, Co, Fe), and has higher capacity and better cyclic stability than LiMn₂O₄. There are various processing methods to prepare nanostructured vanadium pentoxide with high electrochemical performance for lithium ion batteries: self-assembled V₂O₅ hollow microspheres from nanorods; V₂O₅ submicro-belts from sol-gel precursor combined with hydrothermal method; Electrospun V₂O₅ nanofibers; Electrostatic spray-deposited V₂O₅; co-precipitated macro-plates V₂O₅ from water/ethanol media and V₂O₅ nanowires from chemical vapor transport. These nanostructured vanadium pentoxide materials have shown improved electrochemical performance in comparison with conventional cathode materials for lithium ion batteries, however due to the high cost of fabrication and complicated processing method, the broad industrial applications of such nanomaterials are limited.

Therefore, what is desired is an improved method for forming V₂O₅ that provides V₂O₅ films with superior properties when incorporated as cathodes in lithium-ion batteries.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

In one aspect, a V₂O₅ nanosheet is provided having a thickness of from about 3 nm to 1000 nm and a specific surface area of from about 1 m² g⁻¹ to about 500 m² g⁻¹; wherein the V₂O₅ nanosheet comprises a plurality of crystalline domains.

In another aspect, a method of forming polycrystalline V₂O₅ nanosheets is provided. In one embodiment, the method includes the steps of:

-   -   (a) providing a V₂O₅ gel;     -   (b) freezing the V₂O₅ gel to provide a pre-frozen V₂O₅ gel;     -   (c) lyophilizing the pre-frozen V₂O₅ gel to provide lyophilized         V₂O₅; and     -   (d) annealing the lyophilized V₂O₅ to provide polycrystalline         V₂O₅ nanosheets.

In another aspect, a battery is provided comprising a cathode comprising a V₂O₅ nanosheet as disclosed herein.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1. A schematic illustration of the synthesis route of 2D leaf-like V₂O₅ nanosheets according to the disclosed embodiments.

FIGS. 2A-2D. Low- (FIG. 2A) and high-magnification (FIG. 2B) FESEM images of a V₂O₅ nanosheet; FIG. 2C: TEM image of a V₂O₅ nanosheet; FIG. 2D: high resolution TEM (HRTEM) image of an area of FIG. 2C.

FIGS. 3A and 3B. FIG. 3A: XRD pattern of leaf-like V₂O₅ nanosheets. The vertical lines on the x-axis correspond to the standard XRD reflections of orthorhombic V₂O₅ and the inset shows crystalline structure of layered V₂O₅. FIG. 3B: N₂ adsorption/desorption isotherm and corresponding BJH pore-size distribution curves (inset) of leaf-like V₂O₅ nanosheets.

FIGS. 4A-4D. FIG. 4A: CV curves of the first two cycles of leaf-like V₂O₅ nanosheet electrodes at a scan rate of 0.2 mV s⁻¹. FIG. 4B: Discharge capacities of leaf-like V₂O₅ nanosheet electrodes at various current densities. FIG. 4C: Charge/discharge curves of leaf-like V₂O₅ nanosheet electrodes at various current densities. FIG. 4D: Cycling performance of leaf-like V₂O₅ nanosheet electrodes at a current density of 500 mA g⁻¹. Inset shows the charge/discharge curves corresponding to different cycles.

FIGS. 5A and 5B. FIG. 5A: Nyquist plots of leaf-like V₂O₅ nanosheet electrodes measured at various depths of discharge (DOD). FIG. 5B: The calculated R_(sf)+R_(ct) values at the different depth of discharge states.

FIGS. 6A and 6B. Low- (FIG. 6A) and high-magnification (FIG. 6B) FESEM images of V₂O₅ nanosheets.

FIG. 7. XRD pattern of V₂O₅ nanosheets.

FIG. 8. Ragone plot of 2D leaf-like V₂O₅ nanosheet electrodes.

DETAILED DESCRIPTION

Improved V₂O₅ materials are disclosed herein in the form of 2D leaf-like nanosheets. Methods of forming the V₂O₅ nanosheets and batteries (e.g., lithium-ion) incorporating the V₂O₅ nanosheets are also provided.

The particulars shown herein are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of various embodiments of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for the fundamental understanding of the invention, the description taken with the drawings and/or examples making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

The following definitions and explanations are meant and intended to be controlling in any future construction unless clearly and unambiguously modified in the following examples or when application of the meaning renders any construction meaningless or essentially meaningless.

As used herein and unless otherwise indicated, the terms “a” and “an” are taken to mean “one”, “at least one” or “one or more”. Unless otherwise required by context, singular terms used herein shall include pluralities and plural terms shall include the singular.

V₂O₅ Nanosheets

In one aspect, a V₂O₅ nanosheet is provided having a thickness of from about 3 nm to 1000 nm and a specific surface area of from about 1 m² g⁻¹ to about 500 m² g⁻¹; wherein the V₂O₅ nanosheet comprises a plurality of crystalline domains.

The nanosheets are referred to herein as “2D” and/or “leaf-like” due to their unusual shape in the form of sheets. The nanosheets are not formed from a single crystalline domain, but are polycrystalline. This polycrystallinity results from the formation of the nanosheets from annealed aggregates of nanoribbons or nanorods of V₂O₅.

The form of V₂O₅ as nanosheets is beneficial to their use as electrode material (e.g., as cathodes in LIBs). Particularly, the nanosheets provide large surface area, but are thin enough to permit transfer of ions through the sheet. The polycrystalline nature of the nanosheets enhances the ion permeability by providing a looser packing of the V₂O₅ lattice compared to a single-crystalline material.

In one embodiment, the V₂O₅ nanosheets have a thickness of from about 3 nm to 1000 nm. In one embodiment, the V₂O₅ nanosheets have a thickness of from about 3 nm to 100 nm. In one embodiment, the V₂O₅ nanosheets have a thickness of from about 60 nm to 80 nm. The thickness of V₂O₅ can be controlled by tuning the concentration of the V₂O₅ sol. The higher concentration of the V₂O₅ sol during synthesis, the thicker the V₂O₅ nanosheets.

As noted above, the relatively thin nanosheets provide high surface area but facilitate ion transfer across the thickness of the sheet, which provides benefits when incorporated into LIBs.

In one embodiment, the nanosheets have a specific surface area of from about 1 m² g⁻¹ to about 500 m² g⁻¹. In one embodiment, the nanosheets have a specific surface area of from about 25 m² g⁻¹ to about 30 m² g⁻¹. Specific surface area can be obtained through Brunauer-Emmett-Teller (BET) testing. The specific surface area is an important characteristic for LIB electrode materials because it determines the contact area between an active material and an electrolyte in a LIBs; and it determines the number of reaction sites of lithium ion intercalation/deintercalations. The large specific surface area also determines the surface energy and, thus, affects the phase transition involved with lithium ion intercalation/deintercalation process

In one embodiment, the plurality of crystalline domains are orthorhombic. The orthorhombic phase of V₂O₅ is preferred because batteries formed using orthorhombic V₂O₅ nanosheets have extraordinarily high energy density, power density, and capacity.

In one embodiment, the plurality of crystalline domains comprise V₂O₅ nanorods. The nanosheet structure include nanorod-shaped sub-domains due to the method of fabrication, as discussed below. The nanorod shape of the V₂O₅ sub-domains provides the basis for the polycrystallinity of the nanosheets that results in the extraordinary properties of the provided nanosheets.

In one embodiment, the nanosheet has a power density from about 20 W kg⁻¹ to 16,000 W kg⁻¹. In one embodiment, the nanosheet has a power density from about 100 W kg⁻¹ to 8500 W kg⁻¹.

In one embodiment, the nanosheet has an energy density from about 50 Wh kg⁻¹ to 1500 Wh kg⁻¹. In one embodiment, the nanosheet has an energy density from about 200 Wh kg⁻¹ to 900 Wh kg⁻¹.

FIG. 8 is a Regone plot that illustrates exemplary energy and power density data obtained from nanosheets in accordance with the disclosed embodiments. See the EXAMPLES below for a further discussion of FIG. 8.

In one embodiment, the nanosheet further includes a continuous carbon network throughout the nanosheet. The carbon coating is very thin, with a thickness of 0.5 nm to 20 nm. The carbon coatings are either nanocrystalline or amorphous, and either dense or porous. By incorporating a continuous carbon network into the nanosheet, the electrochemical performance of the nanosheets can be improved. The continuous carbon network of the V₂O₅ nanosheet can improve the electrical conductivity of V₂O₅ and affect the thermodynamics and enhance the kinetics of lithium insertion/extraction in LIBs. In addition, the continuous carbon network can also increase the stability of the V₂O₅ material by reducing the surface reactions of V₂O₅ with the electrolyte and accommodating volume variation during charge-discharge cycling.

In one embodiment, the nanosheet further includes a dopant selected from the group consisting of Mn, Fe, Ni, Co, Cr, Ag, Ti, Zn, Sn, Cu, Al, K, Mg, Ca, B, Bi, and combinations thereof. Such dopants can be added to the gel so as to improve the performance of the eventual orthorhombic V₂O₅ film formed. Dopants may improve the characteristics of the V₂O₅ films in a number of ways. First, dopants can improve electrical conductivity, such that doped films have improved Li-ion intercalation properties, such as rate capability and cyclic stability. Second, dopants can serve as nucleation sites that facilitate film formation during electrodeposition. Third, dopants can impede crystallite growth during annealing, such that the crystallite size of doped films can be smaller than undoped films. Fourth, dopants will facilitate the phase transition during lithium ion intercalation and deintercalation, and thus enhance energy storage capacity. Finally, the incorporation of appropriate dopants can improve the cyclic stability.

In another aspect, a battery is provided comprising a cathode comprising a V₂O₅ nanosheet as disclosed herein. In one embodiment, the battery is a lithium ion battery. The integration of materials such as the nanosheets into batteries as cathode materials is well known to those of skill in the art. An exemplary LIB is fabricated and characterized in the EXAMPLES section below.

V₂O₅ Nanosheet Fabrication Method

In one aspect, a method of forming polycrystalline V₂O₅ nanosheets is provided. In one embodiment, the method includes the steps of:

-   -   (a) providing a V₂O₅ gel;     -   (b) freezing the V₂O₅ gel to provide a pre-frozen V₂O₅ gel;     -   (c) lyophilizing the pre-frozen V₂O₅ gel to provide lyophilized         V₂O₅; and     -   (d) annealing the lyophilized V₂O₅ to provide polycrystalline         V₂O₅ nanosheets.

The method is a simple, green approach to V₂O₅ nanosheet growth, as illustrated in FIG. 1. The method begins with the step of providing a V₂O₅ gel. Methods of forming such gels are known to those of skill in the art. An exemplary method of providing a V₂O₅ gel is to combine a V₂O₅ powder (e.g., commercially available) that is reacted with H₂O₂ in combination with ultrasonic treatment to generate the V₂O₅ gel.

The method continues with a step of freezing the V₂O₅ gel to provide a pre-frozen V₂O₅ gel. The freezing step does not take place under vacuum and is referred to as “pre-freezing” because it precedes the lyophilizing (freeze-drying) step. The freezing step is performed at a sufficiently low temperature and for a sufficient amount of time so as to freeze the V₂O₅ gel. In one embodiment, the freezing step lasts at least one day at −20° C.

The method continues with a step of lyophilizing the pre-frozen V₂O₅ gel to provide lyophilized V₂O₅. The time, temperature, and vacuum of this step are sufficient to lyophilize the pre-frozen V₂O₅ gel. In one embodiment, the lyophilizing step is a temperature of −50° C. or less and a vacuum of 0.1 Torr or less.

The method concludes with a step of annealing the lyophilized V₂O₅ to provide polycrystalline V₂O₅ nanosheets. In one embodiment, annealing comprises heating to a temperature of at least 350° C. If the annealing temperature is below 350° C., pure orthorhombic phase V₂O₅ cannot be obtained.

In one embodiment, the V₂O₅ gel comprises a dopant selected from the group consisting of Mn, Fe, Ni, Co, Cr, Ag, Ti, Zn, Sn, Cu, Al, K, Mg, Ca, B, Bi, and combinations thereof. Accordingly, an additional step of the method is to provide a dopant to the V₂O₅ gel. As noted previously, dopants can be used to provide additional properties (e.g., conductivity).

In one embodiment, providing the V₂O₅ gel comprises sonicating a solution of hydrogen peroxide and V₂O₅ powder. In one embodiment, the V₂O₅ powder and the hydrogen peroxide are present to provide a ratio of n(H₂O₂):n(V) of about 8:1.

In one embodiment, the annealing step is performed in an atmosphere selected from the group consisting of air, oxygen, nitrogen, carbon monoxide, carbon dioxide, argon, and combinations thereof. This treatment can be used to tailor the valence state of the vanadium ions and oxygen vacancies in the final V₂O₅ nanosheets. For example, when annealed in air, very little V⁴⁺ can be retained in the final V₂O₅ nanosheets because most of the V⁴⁺ is oxidized during the process of annealing. When annealed in oxygen free atmosphere (nitrogen), most of the V⁴⁺ and oxygen vacancies in cryogel can be retained in the final V₂O₅ nanosheets. When annealed in reducing atmosphere (carbon monoxide), more V⁴⁺ and/or V³⁺ will be generated in the V₂O₅ nanosheets. This low valence state vanadium ions and the accompanied oxygen vacancies play very important roles in modifying lithium-ion storage capability and electron conductivity of the V₂O₅.

In one embodiment, the method further includes a step of adding a carbon source to the solution and performing the annealing step in an oxygen-free atmosphere to provide polycrystalline V₂O₅ nanosheets with a continuous carbon network throughout the nanosheets. During the sol-gel process, carbon sources (organic molecules, such as glucose) can be introduced. In the following annealing process in oxygen free atmosphere, organic molecules will be decomposed and produce carbon coatings on the surface of V₂O₅ nanosheets. Because V₂O₅ is a poor electron conductor, the electrochemical performance of V₂O₅ nanosheets could be significantly improved by carbon coatings. The carbon coating is very thin, with a thickness of 0.5 nm to 20 nm. The carbon coatings are either nanocrystalline or amorphous, and either dense or porous.

The following examples are intended to illustrate, and not limit, the embodiments disclosed herein.

EXAMPLES 2D Leaf-Like V₂O₅ Nanosheets

The method of forming the nanosheets is a simple, green approach, as illustrated in FIG. 1. The method begins with a V₂O₅ powder (e.g., commercially available) that is reacted with H₂O₂ in combination with ultrasonic treatment to generate V₂O₅ gel.

The V₂O₅ gel is optionally diluted, frozen, lyophilized (i.e., freeze-dried), and annealed to obtain V₂O₅ nanosheets. A comprehensive description of the exemplary method is described below in the Experimental section. The low-cost raw materials (commercial V₂O₅ powder and H₂O₂ can be used) and facile experimental procedures favorably enable the method suitable for large-scale production.

FIGS. 2A-2D present FESEM and TEM images of the V₂O₅ nanosheets annealed at 450° C. for 1 h in air. It can be clearly seen that the prepared V₂O₅ has a large-area 2D leaf-like structure (FIG. 2A). The thickness of the V₂O₅ nanosheets is 60-80 nm (FIG. 2B). To the best of our knowledge, this is the first time that such large-area 2D leaf-like V₂O₅ nanosheets have been obtained. From the SEM image shown in FIG. 2B, one can find that the V₂O₅ nanosheet actually is polycrystalline and consists of small nanorods. The TEM image (FIG. 2C) further confirms the 2D sheet structure of the prepared V₂O₅. The high resolution (HR) TEM image (FIG. 2D) of the V₂O₅ nanosheet displays clear lattice fringes with a spacing of 0.26 nm and is indexed to the (310) planes of orthogonal V₂O₅ (JCPDS card No. 41-1426).

Without being bound by theory, the mechanism for the formation of this 2D V₂O₅ nanosheet can be described as follows. During the process of freeze drying, the ribbon-like V₂O₅ fibers intertwine into sheets V₂O₅ during the removal of solvent from the V₂O₅ gel. The freeze-dried V₂O₅ cryogel consists of long nanobelts that are less than 100 nm wide (FIGS. 6A and 6B). The nanobelt morphology could be related to the formation of hydrous V₂O₅ as previously reported in the literature, and this is in good agreement with the XRD diffraction results (FIG. 7). When the V₂O₅ cryogel was annealed in air at 450° C., the hydrous V₂O₅ nanobelts grew into small nanorods and formed orthorhombic leaf-like V₂O₅ nanosheets.

FIG. 3A shows the XRD pattern of the V₂O₅ nanosheets annealed at 450° C. for 1 h in air. All diffraction peaks can be indexed to an orthorhombic phase V₂O₅ (JCPDS card No. 41-1426) with the lattice parameters of a=11.488 Å, b=3.559 Å, c=4.364 Å, agreeing well with literature values. No secondary phase was observed. The orthorhombic phase V₂O₅ has a layered structure consisting of V₂O₅ layers stacking along the c-axis (inset of FIG. 3A). Nitrogen adsorption/desorption isotherm measurements were carried out and the results are shown in FIG. 3B. The Barrett-Joyner-Halenda (BJH) pore size distribution obtained from the isotherm revealed that the sample contains relatively mesoscale pores. The Brunauer-Emmett-Teller (BET) specific surface area has been estimated to be 28 m² g⁻¹.

FIG. 4A presents the cyclic voltammetry (CV) profiles of 2D leaf-like V₂O₅ nanosheet electrodes for the first two cycles at a scan rate of 0.2 mV s⁻¹. In the first cycle, the three intensive reduction peaks located at 3.30, 3.08, and 2.17V, corresponding to the phase transitions α/ε, ε/δ, and δ/γ, respectively. Three obvious oxidation peaks appeared during anodic scanning, at 2.57, 3.35, and 3.48 V, respectively. An additional cathodic peak observed in the high potential region (at 3.56 V) could be ascribed to the irreversible phase transition of the γ/γ′ system. The symmetrical features of the CV curve suggest good reversibility of the cycling process. FIG. 4B gives the cycling response of 2D leaf-like V₂O₅ nanosheet electrodes at various charge/discharge current densities. The discharge capacities measured in the voltage window from 2.0 V to 4.0 V are 303, 273, 251, 219, and 160 mA h g⁻¹ at current densities of 50, 200, 500, 1000, and 2000 mA h g⁻¹, respectively. Even at a very high current density of 5000 mA g⁻¹, the 2D leaf-like V₂O₅ nanosheet electrode can still deliver high capacity of 104 mA h g⁻¹. Note that this rate capability is better than those of carbon-coated V₂O₅ and other V₂O₅ nanostructured electrodes reported in literature to date (Table 1).

TABLE 1 A comparison of C-rate results between electrodes of the present disclosure (“Exemplary Electrodes”) and other reported V₂O₅ based electrodes. All data are second discharge capacities. Samples discharge capacity/mAh g⁻¹ Current Exemplary Ref. Ref. Ref. Ref. Ref. density Electrodes 1 2 3 4 5  50 mA g⁻¹ 303 250 120 mA g⁻¹ 261 200 mA g⁻¹ 273 500 mA g⁻¹ 251 140 580 mA g⁻¹ 135 735 mA g⁻¹ 146 1000 mA g⁻¹  219 190 1764 mA g⁻¹  103 2000 mA g⁻¹  160 150 Ref. 1: J. Liu, H. Xia, D. Xue, L. Lu, J. Am. Chem. Soc. 2009, 131, 12086-12087. Ref. 2: A. Sakunthala, M. V. Reddy, S. Selvasekarapandian, B. V. R. Chowdari, P. Christopher Selvin, Energy Environ. Sci. 2011, 4, 1712-1725. Ref. 3: J. Liu, Y. Zhou, J. Wang, Y. Pan, D. Xue, Chem. Commun. 2011, 47, 10380-10382. Ref. 4: S. Wang, Z. Lu, D. Wang, C. Li, C. Chen, Y. Yin, J. Mater. Chem. 2011, 21, 6363-6369. Ref. 5: Y. Wang, H. J. Zhang, K. W. Siah, C. C. Wong, J. Lin, A. Borgna, J. Mater. Chem. 2011, 21, 10336-10341.

The results described in the present disclosure show that the 2D leaf-like V₂O₅ nanosheet structure favorably reduces the diffusion length for lithium ions and enables high-rate performance of LIBs. With the charge/discharge current density increasing from 50 to 5000 mA g⁻¹, the powder density increases from 142 W kg⁻¹ to 8410 W kg⁻¹ (FIG. 8). These compare to supercapacitors, which are addressing the extremes of power-density needs (˜1000-20000 W kg⁻¹) of commercially available devices, though their energy density is only about 1-20 Wh kg⁻¹. The 2D leaf-like V₂O₅ nanosheets may be used for novel and superior electrochemical energy-storage devices with both high-power and high-energy densities. FIG. 4C presents the charge/discharge curves of the 2D leaf-like V₂O₅ nanosheet electrodes at various current densities in the range 2.0-4.0 V. Reversible plateau regions can be observed at all the current densities. The discharge/charge plateaus agree well with the redox peaks shown in the CV curve of FIG. 4A. With an increase in current density, especially at very high current densities (2000 and 5000 mA g⁻¹), the discharge voltage decreases and the charge voltage increases due to an increasing polarization effect. FIG. 4D shows the cycling performance of the 2D leaf-like V₂O₅ nanosheet electrodes at a current density of 500 mA g⁻¹. After 100 cycles, a specific discharge capacity of 206 mAh g⁻¹ can be retained. The capacity fading rate is about 0.22% per cycle, which is lower than the results reported for this material. The 2D leaf-like V₂O₅ nanosheet electrodes maintained a well-defined reversible plateau region even at the 60^(th) cycle. It is noticeable that the capacity loss with the plateau of about 3.15 V is much larger than those of others. Therefore, it can be inferred that leaf-like V₂O₅ processes relatively poor reversibility for lithium ion intercalation/de-intercalation with the voltage plateau of about 3.15 V, which is considered a main reason for capacity fading. The excellent high-rate performance of leaf-like V₂O₅ nanosheet electrodes is believed to be based on their unique architecture results from at least the following aspects: The large specific area of the 2D leaf-like V₂O₅ nanosheets facilitate the electrolyte to transport the intercalation and de-intercalation of the lithium ions; and the hierarchical porous structure of the V₂O₅ nanosheets relax the mechanical strain generated upon the charge/discharge cycling.

FIG. 5A provides the Nyquist plots of the 2D leaf-like V₂O₅ nanosheet electrode at various depth of discharge (DOD) after the electrode was activated at 20 mA g⁻¹ for 4 cycles. The semicircle in the high frequency region relates to the combined process of surface film (R_(sf)) and the charge transfer resistance (R_(ct)). The low frequency semicircle (for 76%, 90%, and 100% DOD) corresponds to a bulk phenomenon, which arises from electronic conductivity of active material and ionic conductivity of the electrolyte filled in the pores of composite electrode. FIG. 4B presents the calculated R_(sf+ct) values (diameter of semicircle at high frequency) under various DOD states. With DOD increasing from 0% to 62%, the R_(sf+ct) value initially decreases from 853Ω to 637Ω at 28% DOD and then slightly increases to 660Ω at 62% DOD. Further increasing DOD from 76% to 100% leads the R_(sf+ct) value abruptly increased from 740Ω to 1660Ω. Such a large increase of R_(sf+ct) value suggests that the electrochemical reaction under high DOD became much more difficult than under low DOD, due to the change of phase structure. Another important feature of the EIS plots is the appearance of bulk resistance (R_(b), the second semicircle at low frequency, FIG. 8) under high DOD (76%, 90%, and 100% DOD) states. This suggests that under those states the electrode is a poor electronic conductor. While under low DOD (from 0% DOD to 62% DOD) states, the bulk resistance (R_(b)) is negligibly small, indicating that electrodes are good electronic conductors. This result is in good agreement with the four-probe current-voltage characteristics for Li_(x)V₂O₅ nanoribbons reported previously. Large changes in R_(b) values have also been observed in some other cathode materials, depending on the DOD, and are attributed to reversible semiconductor-metal transformations during cycling.

In summary, we have developed a facile, green, and low-cost synthesis of 2D leaf-like V₂O₅ nanosheets. The unique nanoscale characteristics, including 2D morphology, hierarchical porous structure, large specific surface, of these 2D V₂O₅ nanosheets leads to the superior electrochemical performance in terms of their specific capacity, rate capability, and cyclability when they are used as cathode material for LIBs. The obtained excellent performance opens up new opportunities in the development of high performance next-generation LIBs.

Experimental

Material Preparation:

Vanadium pentoxide gel was prepared using a known method. In brief, V₂O₅ powders (99.8%, Alfa-AESAR) were added into de-ionized water and H₂O₂ (30 wt. % in H₂O, Sigma-Aldrich) to form a solution with a V₂O₅ concentration (C_(V)) of 0.3 M and n(H₂O₂):n(V) of 8:1. The resulting solution was stirred for 15 min while kept in a water bath at a room temperature and then sonicated for 15 min for the reactions. This solution was later diluted to C_(V)=0.056 M and then sonicated for about 80 min until the solution turned into brownish red V₂O₅ gel. This gel was further dispersed and diluted to a C_(V) of 0.03 M, and stirred in de-ionized water until a homogenous red-colored, viscous solution was formed. This solution was pre-frozen in a freeze refrigerator at −20° C. for 1 day and then freeze-dried under vacuum at −50° C. for 3 days in a Labconco FreeZone 1 L freeze dryer. After drying, the V₂O₅ cryogel was annealed in ambient atmosphere at 450° C. for 1 h to form 2D leaf-like V₂O₅ nanosheets.

Material Characterization:

The phase structure and morphology of the as-prepared samples were characterized by X-ray diffraction (XRD, Philips 1820 X-ray diffractometer), field emission scanning electron microscopy (FESEM, JEOL, JSM-7000), and transmission electron microscopy (TEM, Tecnai G2 F20 S-Twin). The Brunauer-Emmett-Teller (BET) specific surface areas and pore size distributions were measured with QuantaChrome NOVA 4200e analyzer (working gas N₂, 77 K).

Electrochemical Measurements:

The electrochemical properties of the 2D leaf-like V₂O₅ nanosheets were tested in coin-cells with metallic lithium as the anode and polypropylene (PP) film as separator. The coin-cells were assembled in an argon-filled glove-box. The cathodes were fabricated by mixing V₂O₅ nanosheets, super P carbon black, and poly(vinyldifluoride) (PVDF) at a weight ratio of 70:20:10 in n-methyl-2-pyrrolidone (NMP) solvent. The resulting mixture was then uniformly spread on an aluminum foil current collector. Finally, the electrode was dried at 80° C. for 12 h. The electrode loading was about 2 mg cm⁻². The electrolyte solution was made of 1 M LiPF₆ in a 1:1 (V:V) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The cells were galvanostatically charged and discharged under different current densities between 2.0 V and 4.0 V (vs Li/Li⁺) using Arbin BT-2000 battery tester at room temperature. Cyclic voltammetry (CV) studies were carried out on an electrochemical workstation (CHI 605 C) between 2.0 and 4.0 V at a scan rate of 0.2 mV s⁻¹. Electrochemical impedance spectroscopies (EIS) were performed using the Solartron 1287A in conjunction with a Solartron 1260FRA/impedance analyzer. In EIS measurement, the frequency ranged from 100 kHz to 5 mHz and the AC amplitude was 5.0 mV.

While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. 

The invention claimed is:
 1. A method of forming polycrystalline V₂O₅ nanosheets comprising the steps of: (a) providing a V₂O₅ gel by sonicating a solution of hydrogen peroxide and V₂O₅ powder; (b) freezing the V₂O₅ gel to provide a pre-frozen V₂O₅ gel; (c) lyophilizing the pre-frozen V₂O₅ gel to provide lyophilized V₂O₅; and (d) annealing the lyophilized V₂O₅ by heating to a temperature of at least 350° C. to provide polycrystalline V₂O₅ nanosheets, wherein the hydrogen peroxide and the V₂O₅ powder are present in a molar ratio of H₂O₂:V₂O₅ of about 16:1.
 2. The method of claim 1, wherein the freezing step lasts at least one day at 20° C.
 3. The method of claim 1, wherein the lyophilizing step is a temperature of 50° C. or less and a vacuum of 0.1 Torr or less.
 4. The method of claim 1, wherein the annealing step is performed in an atmosphere selected from the group consisting of air, oxygen, nitrogen, carbon monoxide, carbon dioxide, argon, and combinations thereof.
 5. The method of claim 1, further comprising adding a carbon source to the solution and performing the annealing step in an oxygen-free atmosphere to provide polycrystalline V₂O₅ nanosheets with a continuous carbon network throughout the nanosheets.
 6. The method of claim 1, wherein the V₂O₅ gel comprises a dopant selected from the group consisting of Mn, Fe, Ni, Co, Cr, Ag, Ti, Zn, Sn, Cu, Al, K, Mg, Ca, B, Bi, and combinations thereof. 